In electrohydraulic systems that include a plurality of electrohydraulic devices, such as servo actuators, motors and pumps, it is conventional practice to couple all of such devices to a remote master controller for coordinating or orchestrating device operation to perform a desired task. Motors and actuators may be employed, for example, at several coordinated stages of a machine tool line for automated transfer and machining of parts at a series of work stations. In accordance with conventional practice, the master controller may comprise a programmable controller or the like coupled through individual digital-to-analog converters to the various remotely-positioned electrohydraulic devices for supplying control signals thereto. For closed-loop operation, a sensor is positioned at each electrohydraulic device for sensing operation thereof, and feeding a corresponding signal to the master controller through an analog-to-digital converter.
Thus, in a system that embodies a plurality of electrohydraulic devices, a substantial quantity of electrical conductors must be provided for feeding individual control signals to the various devices and returning sensor signals to the master controller. Such conductors interfere with system design and operation, and are subject to failure. The bank of d/a and a/d converters for feeding signals from and to the master controller add to the expense and complexity of the overall system. Perhaps most importantly, system performance is limited by capabilities of the master controller. For example, a programmable controller may require one hundred milliseconds to scan a device sensor signal, compute a new control signal and transmit such control signal to the remote device. Such overburdened programmable controller operations are not acceptable in high performance applications that may require a six millisecond response time, for example, at each of a plurality of remote devices.
It is therefore a general object of the present invention to provide an electrohydraulic servo system that exhibits the fast response time necessary for high performance applications, while at the same time reducing cost and complexity that inhere in prior art system of the character described above. In furtherance of the foregoing, a more specific object of the invention is to provide a system of the described character wherein each of the system electrohydraulic devices embodies microprocessor-based control adapted to communicate with a central or master controller and for thereby distributing control of the several electrohydraulic devices while maintaining overall coordination thereamong.
Conventional in-line variable displacement pumps comprise a case or housing within which a cylinder block is coupled to a rotatable drive shaft. The cylinder block contains a plurality of cylinder cavities disposed in a circumferential array around the shaft axis. A corresponding plurality of pistons are slidably positioned within the respective cylinders. The pistons engage a yoke cam that is variably positionable within the pump housing for collectively adjusting stroke or displacement of the pistons within the cylinders. The cylinder block rotates against a valve plate having arcuate inlet and outlet kidney-shaped slots that serve in a well-known manner to provide properly phased or timed communication between the end ports of the cylinder bores within which the pistons reciprocate, and inlet and outlet passages and ports in the pump housing. The yoke is variably positionable by an actuator piston that is hydraulically coupled to a control valve mounted on the pump adapter or valve block.
In electronically controlled pumps, the control valve comprises a solenoid valve that is connected to separate control electronics for variably controlling flow of hydraulic fluid through the solenoid valve, and thereby controlling position of the actuator piston and yoke. U.S. Pat. No. 4,823,552 discloses a variable displacement hydraulic pump and a control system in which a plurality of sensors responsive to operating conditions at the pump, such as pump speed, output pressure, output flow, yoke displacement and pump temperature, are connected to an electronic pump controller. The controller, which preferably comprises microprocessor-based control electronics, receives pump command signals from a remote master controller, compares the command signals with sensor feedback indicative of pump operating conditions, and controls pump displacement through a solenoid valve as a function of a difference or error therebetween. Conventionally, the sensors and controllers are provided as components separate from the pump itself, and must be assembled and interconnected (as previously described) at the installation site. The complexity of installation often delays start-up and limits applications.
In microprocessor-based control of hydraulic pump or motor speed, such as that in the U.S. application noted in the preceding paragraph, one method of sensing unit speed is to measure the time interval between adjacent teeth of a toothed wheel or sprocket affixed to the shaft. Typically, this is accomplished through use of an electromagnetic pickup and feeding the cyclic electronic signal through appropriate conditioning circuitry to the control microprocessor. Each tooth passing the pickup causes the microprocessor to interrupt its computations and other control programming to store and then reset the value of the pickup internal timer. A number of limitations and problems are encountered in use of this technique for measuring the speed of the pump or motor. For example, at high speeds, resolution is limited due to the short time interval between adjacent teeth, necessitating that the interval be averaged over a substantial number of tooth intervals. Furthermore, long and/or frequent interrupt service routines are undesirable in such applications because they delay the servicing of other interrupts and normal control processing. This can result in loss of data, noise on the pulse width modulated valve control signal, and other problems. Moreover, the variable time delay caused by servicing other interrupts can result in errors in measuring the interval between detected teeth.
U.S. Pat. Nos. 4,744,218 and 4,811,561 disclose electrohydraulic servo systems in which a plurality of electrohydraulic devices, including a variable displacement pump and associated microprocessor-based controller, are connected in common by a serial communication bus to a remote master controller. The pump controller includes address switches for selecting an appropriate address for communications with the master controller, and memory in which a plurality of control programs are stored for remote selection by the master controller. U.S. Pat. No. 4,757,747 discloses an electrohydraulic system that includes a plurality of electrohydraulic devices, specifically a plurality of actuator-controlling servo valves, individually controlled by on-board microprocessor-based control electronics. Each individual device controller is connected to and addressable by a master controller for coordinating operation of the various devices.
A general object of the present invention is to provide an improved rotary electrohydraulic machine, particularly a variable displacement hydraulic pump, in which all control components, including pump condition sensors and microprocessor-based control electronics, are fully integrated into a single compact inexpensive package, and which may be readily employed in a wide variety of system applications. Another object of the invention is to provide a rotary electrohydraulic machine of the described character with enhanced commonality of hardware, including particularly control sensors, electronics and wiring, among machines of varying size and rating.
A further object of the invention is to provide a system for controlling operation of a rotary hydraulic machine, such as a pump or motor, in which machine speed is monitored by state-of-the-art microprocessor-based control with reduced interrupt service problems, and with enhanced measurement accuracy and resolution.
Yet another object of the present invention is to provide a system for controlling an electrohydraulic valve, such as the displacement-control valve of a variable displacement pump, with valve drive electronics that exhibit reduced heat dissipation as compared with prior art devices of a similar character, and that have enhanced capabilities for protecting the drive circuitry against damage due to failure at the valve solenoid.